1. Fiedl of the Invention
This invention is directed to an electronic ballast system for fluorescent or gas discharge lamps. In particular, this invention relates to a universal electronic ballast system for fluorescent lamps. Still further, this invention relates to an electronic ballast system which operates over a wide voltage input range at either 50 or 60 cycles, and may be utilized for driving fluorescent lamps having any one of a plurality of wattage ratings, tube diameters and lengths. Further, this invention is directed to an electronic ballast system utilizing a switching power supply which draws a substantially constant sinusoidal current from the AC power source. More in particular, this invention pertains to the induction circuit being coupled in feedback relationship to the switching circuit for terminating the switching circuit operation responsive to the gas discharge lamp being electrically uncoupled from the output transformer.
Still further, this invention directs itself to a switching circuit wherein both the load current and switching transistor collector current are monitored to provide positive feedback to the base drive circuit. Additionally, this invention pertains to a switching circuit wherein the emitter current of the switching transistor is monitored for rapidly turning off the switching transistor responsive to the emitter current reaching a predetermined value. The emitter current monitoring circuit providing a means by which the switching circuit compensates for transistor characteristics which vary from one switching transistor to another. Further, the switching circuit is feedback coupled to the regulated power supply circuit for terminating the generation of the boost voltage provided thereby, responsive to electrical uncoupling of the gas discharge lamp from the ballast system.
2. Prior Art
Electronic ballast systems for gas discharge or fluorescent lamps are well known in the art. However, in some prior art electronic ballast systmes, removal of the gas discharge of fluorescent lamp from the ballast circuit causes exceswsive voltage outputs to the lamp connection contacts. This condition can have a deleterious effect on the operating life of the ballast system components.
Other prior art systems compensate for the no-load condition by incorporating complex inductigve circuits whose impedance varies inversely proportional to the load current, or alternately shift the operting frequency of the ballast system to force a lower voltage to be generated. However, such systems are difficult to manufacture, requiring tight controls on component characteristics. Problems occur where some of the critical components cannot be maintained within the tight tolerances required and thus some percentage of such ballast systems do no function sufficiently well to provide the necessary no-load protection.
Other prior art electronic ballast systems may be designed to operate over a range of input voltages, without the requirement for changing transformer taps, or component values, such systems are designed to drive a particular wattage lamp. Whereas in the instant invention not only will the ballast operate on a wide range of AC voltages, but lamps of any one of a wide range of wattages, tube diameters and lengths may be efficiently operated with the instant invention. This improvement provides great advantages to manufacturers of lighting systems wherein a single ballast system is usable within a broad range of lighting fixtures, as opposed to prior art systems which required a particular ballast to be matched with a lamp of particular wattage and physical characteristics.